Velocity modulated electron tube with integrated focusing and getter pump systems, the pump having multiple getter-coated electrodes



- VELOCITY MODULATED ELECTRON TUBE WITH INTEGRATED Focusme AND GETTER PUMP SYSTEMS THE PUMP HAVING MULTIPLE GETTER-COATED ELECTRODES Filed May 7, I960 Q 2 Sheets-Sheet} .f-"J;n-. 17,-1967 W.VEI'IL'HLETALL 3,299,311

' Fi led May 7. 1960 VELOCITY MQDULATED ELECTRON TUBE W ITH INTEGRATED FOCUSING AND GETTER PUMP SYSTEMS, THE PUMP HAVING MULTIPLE GETTER-COATED ELECTRODES 2 Sheets-Sheet 2 United States Patent C) 3 299 311 VELOCITY MODULATEDELECTRON TUBE wrrrr INTEGRATED FocUsrNG AND GETTER PUMP SYSTEMS, THE PUMP HAVING lVllUlLTliPLE This invention disclosed herein is concerned with a travelling wave tube comprising a system of electrodes arranged in a homogeneous or periodic magnetic focusing field, especially a travelling field tube having an ion getter p p- 5 t A high quality vacuum is in the case of travelling field tubes of great importance because ion oscillations which are troublesome in the operation of a tube can be avoided and the useful life and storage of a tube with highgrade vacuum can be greatly extended. Efforts are therefore being made to provide in the production of such tubes a very good vacuum, amounting, for example, to at least torr and to maintain such vacuum in the finished tubes. High power tubes have been provided with an ion pump so as to assure satisfactory vacuum even under stressed operating conditions.

Ion getter pumps as separate units as well as in direct connection with electron discharge systems are known. The operation thereof is based upon ionization of residual gases and transfer thereof to an electrode with a well absorbing surface at which they are bound. Pure metals, such as tantalum, niobium, zirconium or titaniumhave been found particularly suitable for the formation of such absorbing surfaces, in the form of getter mirrors, by vaporization or spraying of such metals. Such a getter mirror is continuously or periodically renewed or supplemented in order to maintain and to increase the getter action thereof. The operation of such a getter pump requires voltages of several thousand volts, for the maintenance of an ion discharge path and for the transport of the ions formed; further, at least two electrodes arranged so as to form therebetween an ionization path as long as possible, as well as a strong magnetic field for extending the ion path and therewith increasing the probability for an impact ionization. These three requirements are satisfied in most travelling wave tubes.

' The invention has in view of this situation particular significance for all kinds of travelling wave. tubes, and especially for traveling field tubes, rearward wave oscillators and klystrons, that is, for tubes operating with a I magnetically focused axially extending electron beamof high density.

The object underlying the invention resides in utilizing the electrodes provided for the operation of the respective tube, with the potentials placed thereon and the magnetic fields'thereof, for the functional mechanism of the ion pump. According to the invention, this object is realized in connection with a tube of the previously noted kind, especially a travelling field tube, wherein parts of the electrode system with the potentials thereon and the magnetic focusing field, cooperate in effecting the functions of the ion getter pump, employing as an ion collector, arranged symmetrically in the beam path, near the cathode, at least two electrodes having parts, especially on the surface thereof, which are adapted to operate as getter at least during the operation of the respective tube.

For example, in the case of a travelling field tube, the

accelerated electrons emitted from an electron gun are, with the cooperation of a strong magnetic focusing field guided in a beam of high density along an extended spiral path, usually within a helix, thereby effecting an impact ionization with the residual gases. The ions thereby produced, preponderantly positive ions, are moved, more or less focused, to the cathode, owing to their polarity and the action of the focusing means for the electron beam, and would cause considerable destruction of the cathode by the impact effect.

In known tubes of this kind, the emitting plane of the cathode is for this reason provided with a central opening formed therein, for the passage of positive ions, which are bound at an absorption plane formed in back of the cathode. However, this expedient is in its action imperfect and produces conditions which are in part unfavorable for the discharge operation of the tube.

The focusing devices customarily used for the electron discharge operation generally prevent ions from leaving the cross sectional discharge area which is filled with electrons. The electrodes forming, in the tube described herein, the ion collector, are for this reason so arranged with respect to the electron beam path, that the ions reach such electrodesfby the action of electron optical means, along substantially radially directed paths. The electron optical means required therefor, especially the electrodes forming the ion collector, are of particular configuration, and the corresponding potentials are particularly selected, so that the potential distribution produces upon the axis either a saddle or a strong collector lens for the electron beam, from which the electrical field strength is so oriented that the ions are radially outwardly deflected and accelerated.

This measure has, as compared with the previously customary ion getter pumps, very particular advantages. Previously known ion getter pumps frequently fail upon reaching a given vacuum, because the ion discharge path is disrupted owing to the lack of sufficient ion impact possibilities and because of insufiicient pro-ionization caused, for example, by the even present radiation in height. In the tube described herein, there will always be sufiicient electrons for the pre-ionization, owing to the combination of the ion getter pump with the electron discharge space, and sufiicient possibilities for an impact ionization will always be present due to the spirally extending paths of the discharge carriers, so that a disruption of the discharge path cannot occur. The ions diffusing respectively from the collector space and the cathode space are ripped from the electron beam, owing to the peculiarity of the field. Accordingly, there is produced, within the operation of the ion getter pump mechanism, the action of an ion trap, so that the discharge path proper is continuously liberated of ions.

The required ion collector may be formed, in the simplest case, by an electrode which either consists substantially of a getter material such as titanium or which is in operation continuously or periodically provided with a new titanium coating (mirror) or supplemented with such coating. However, it is also possible to provide an ion collector comprising two separate electrodes. One of these electrodes may be provided with a potential such that the ions reach such electrode with considerable impact velocity, the kinetic energy causing thereby a vaporization of the getter material on such electrode, such vaporized material being deposited on the adjacent second electrode to form the absorbing mirror surface. The potential of the second electrode, which is in operation continuously provided with a new getter mirror, is so selected that it is impacted by the ions with reduced velocity which is however sufiicient for effecting heating thereof to obtain a good getter action for the absorption and binding of the ions. The first electrode of the ion collector is for this purpose particularly advantageously. formed of perforated material, for example, in the form of a mesh cylinder or helix, so that it is impacted only by part of the ions, while the ions passing therethrough impact with reduced velocity the second electrode disposed in back of the first electrode.

The dimensioning of the permeable part of the perforated electrode in conjunction with the respective potential, makes it possible to adjust the ratio of the parts of the passing and impacting ions so as to obtain a sufficient but not excessive vaporization of the getter material at one of the electrodes and at the other electrode a heating which is sufiicient for an approximately optimum getter action.

Further details of the invention will appear in the course of the description which is rendered below with reference to the accompanying drawings showing in purely schematic representation embodiments thereof. Parts which do not directly contribute to the understanding of the invention, such as discharge vessels, magnets, etc., have been omitted in the drawings. Corresponding parts are identically referenced.

FIGS. 1 and 2 show embodiments, for use in connection with travelling wave tubes, in which the ion collector, which is respectively disposed at the beginning and at the end of the helix, is formed by electrodes forming part of the electron discharge system; and

FIGS, 3 to 6 indicate embodiments in which the electrodes which are part of the ion collector form, jointly with the adjacent electrodes of the discharge system, a kind of toroid, whereby a perforated hollow inner cylinder serves, for example, as one electrode of the ion collector while an outer hollow cylinder serves as the second electrode therefor, one of the cylinders being always on cathode potential.

FIG. 1 represents the essential part of an electrode system of a travelling wave tube, for example, a high perveance gun. The high density electron beam 1 which is focused by a not illustrated magnetic field, moves from the cathode 2 which is surrounded by the Wehnelt electrode 3, through the helix 4, tothe right, to a not illustrated collector, thereby causing, along its path, by impact ionization, the formation of ions of positive polarity. The beginning of the helix 4' is by means of a cylinder 5 connected with the acceleration anode 6 which is formed as an apertured diaphragm, thus producing for the ions a strong collector lens, by the action of customary potentials applied, as indicated by the potential lines.

The field is in the neighborhood of the axis so oriented for the positive ions 16, iii, moving in the electron beam toward the cathode, that the ions are deflected and accelerated along approximately radial paths, i.e., transverse to the electron beam direction, for example, to the Wehnelt electrode 3 which is at cathode potential.

The Wehnelt electrode is for this purpose made of getter material, for example, titanium. The ions impact the Wehnelt electrode with considerable velocity, effecting vaporization of the titanium which travels to the neighboring anode on which the greater part of the ions is then absorbed and bound. This arrangement as well as the arrangements illustrated in FIGS. 3 to 6 may be analogously applied, as shown in FIG. 2, to the space helix collector, in a case in which the collector is on a lower potential than the helix.

In the embodiment according to FIG. 2, the helix is terminated by a part 11 which is extended by a hollow somewhat wider cylindrical portion 12 into which projects a perforated cylindrical part 13 forming an extension of the collector 14, a gap being left between the parts 11 and 13. Owing to the fact that the potential of the collector is lower than that of the helix, for example, depending on the type of the tube, by one-half of the potential on the helix, there is formed a strong collector lens in the gap plane, which deflects ions 10, in the electron beam approximately radially outwardly. At the perforated hollow cylinder 13 which consists substantially of titanium, is effected a vaporization of the titanium, owing to the high impact velocity of the ions, the vaporized material being deposited on the adjacent widened extended end 12 of the helix and acting as a getter mirror for the ions.

The advantage of the embodiments represented in FIGS. 1 and 2, as compared with other known arrangements, resides in that no auxiliary electrodes are needed for the getter pump and that the required potential distribution does not constitute detrimental conditions for the electron discharge mechanism as such.

FIG. 3 shows in schematic manner parts of a travelling field tube in which the ion collector 8, 9 is arranged near the cathode, between the beginning of the helix 4 and the acceleration anode 7. The electrodes, forming the ion collector, are constructed and arranged so that they form approximately a toroid of rectangular cross section with circularly shaped end surfaces, the inner hollow cylinder 8 which consists substantially of titanium, through which the electron beam passes axially, being perforated and serving as one electrode and the outer cylinder 9 serving as the other electrode. The inner hollow cylinder 8 extends close to but separated by a small gap from the end walls of the other electrode 9. The helix 4 is at the beginning thereof connected with an apertured diaphragm 6, by way of a cylinder 5, the toroidal ion collector 8, 9 being thus disposed approximately symmetrically between the diaphragm 6 and the acceleration anode 7.

Since the perforated inner cylinder 8, which forms one electrode of the ion collector, is at cathode potential, while the outer hollow cylinder 9, which forms the other electrode of the ion collector, is at a potential which is at least a few hundred volts lower than the helix potential, and since the potential from the outer electrode 9, which is connected with the two end plates, drops in the direction of the acceleration anode 7 while increasing in the direction of the helix 5, 6, there will be produced in operation, a field distribution so that a saddle is formed along the axis within the toroid. The ions 10, 10, formed by impact ionization in the electron beam therefore encounter, at such field distribution, a field direction causing deflection thereof initially approximately radially toward the inner hollow cylinder 8, some of the ions impacting the inner cylinder and effecting, owing to their impact velocity, vaporization of part of the titanium, and others passing through perforations and reaching the outer hollow cylinder which is coated with vaporized titanium. A very effective ion trap is formed owing to the particular field distribution described and especially owing to the formation of a saddle upon the axis, so that ions coming from both directions are ripped from the electron beam, thus effecting a very good getter pump action.

FIG. 4 shows a simplified modification of the arrangement just described with reference to FIG. 3. The modification resides in that the outer electrode 9 of the ion collector is by way of the end plate 6 directly connected with the start of the helix 4. This results in a saving so far as electrodes are concerned, but it also eliminates the collector lens which forms, in the arrangement according to FIG. 3, between the ion collector and the helix. There thus remains only a field distribution forming a saddle within the ion collector, whereby the ions are in the manner of an ion trap deflected radially outwardly to the inner perforated hollow cylinder 8 and then to the outer hollow cylinder 9. The potential of the acceleration anode 7 is by a few hundred volts higher than that of the helix 4, so that the ions cannot move against the resulting field (thus preventing ions from reaching the cathode).

In the arrangement shown in FIGS. 5 and 6, the inner hollow cylinder 8 is connected with the end plates such as 6, which supplement the ion collector to form a toroid, and by way of a cylinder 5 also with the helix- 4. However, the outer hollow cylinder 9 is by narrow gaps separated from the end plates 6 and lies at cathode potential. The potential distribution which is thereby effected produces a field direction which causes the ions 10, appearing in the electron beam 1, to move in part through openings in the perforated inner cylinder 8, with considerable velocity to the outer cylinder 9. The getter material on the outer cylinder 9 is in part vaporized, due to the considerable impact velocity of the ions, and reaches the inner perforated cylinder 8 where the directly impacting ions are bound. In FIG. 5, the acceleration anode 7 is at a potential lying between that of the cathode and the helix, for example, midway thereof.

A considerable improvement of the pump action is obtained by making the potential at the acceleration anode 7 by a few hundred volts higher than that of the helix, thus forming a kind of auxiliary ion trap which assures that no ions can pass through the resulting field and reach the cathode.

Changes may be made Within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

We claim:

1. A system of electrodes arranged in a magnetic focusing field of a velocity modulated tube having an ion getter pump, wherein parts of the electrode system, with the potentials thereon and the magnetic focusing field, cooperate jointly to effect the operation of the getter pump, and comprising an ion collector formed by at least two electrodes arranged externally of and symmetrically with respect to the electron beam path, said electrodes carrying especially on the surface thereof material which is in the operation of the respective tube adapted to function as a getter.

2. A system according to claim 1, wherein said electrodes are arranged with respect to the electron beam path so as to form electron optical means for causing ions to move thereto along paths oriented transversely to the beam direction.

3. A system according to claim :2, wherein said electrodes are arranged and provided with potentials, so that ions impact one of the electrodes with considerable velocity, causing vaporization of getter material, while moving to the other electrode with reduced impact velocity.

4. A system according to claim 2 for a traveling wave tube, comprising a high perveance gun, a Wehnelt electrode disposed ahead of the emission face of the cathode and formed as a fiat frustum of a cone with the widened part thereof facing in a direction of the helix extending along the electron beam axis, and an acceleration anode formed as an apertured diaphragm and connected with the helix, said Wehnelt electrode and said acceleration anode serving as parts of the ion collector.

5. A system according to claim 2, for a traveling wave tube comprising a helix extending along the electron beam axis, a tubular extension connected with the helix and forming one electrode for the ion collector, an electron collector provided with a perforated hollow cylinder extending therefrom and forming the other electrode of the ion collector, said perforated cylinder projecting into said tubular extension and forming a narrow gap therewith, the potential on said cylinder being lower than that on the helix.

6. A system according to claim 2, wherein said ion collector comprises two' tubular concentrically disposed cylindrical electrodes and radially extending circular end plates forming therewith approximately a toroid with rectangular cross section, the inner electrode which is axially transversed by the electron beam being substantially formed of titanium and being perforated.

7. A system according to claim 6, for a traveling Wave tube comprising a helix extending along the electron beam axis, an acceleration anode, an apertured diaphragm connected with the helix, said ion collector being disposed approximately symmetrically between said acceleration anode and said apertured diaphragm, the ends of the inner cylindrical electrode of said toroidal ion collector being spaced from the respective circular end plates by narrow gaps.

8. A system according to claim 7, wherein said perforated inner cylindrical electrode is at cathode potential while the outer cylindrical electrode is at a potential which is a few hundred volts =lower than that of the helix.

9. A system according to claim 6, wherein the outer cylindrical electrode of the ion collector is connected with the helix.

10. A system according to claim 6, where said cylindrical inner perforated electrode is connected with the helix, said outer cylindrical electrode being separated from said circular end plates by narrow gaps and being at cathode potential.

11. A system according to claim 10, wherein the potential on the apertured diaphragm, which serves as a first acceleration anode, lies between the potential of the cathode and the potential of the helix.

12. A system according to claim 11, comprising means for mechanically and electrically connecting said inner cylindrical electrode with the helix, the potential on the acceleration anode being by a few hundred volts lower than that of the helix.

13. A system according to claim 10, wherein the potential on the apertured diaphragm, which serves as a first acceleration anode, is more positive than that of the helix.

14. A system according to claim 12, comprising means for mechanically and electrically connecting said inner cylindrical electrode with the helix, the potential of the acceleration anode being by a few hundred volts higher than that of the helix.

15. A system according to claim 8, wherein the acceleration electrode is at a potential which is lower than that of the outer cylindrical electrode of the ion collector and the potential of the outer cylindrical electrode is lower than the potential of the helix.

References Cited by the Examiner UNITED STATES PATENTS 2,767,344 10/1956 Hines 3l53.5 3,073,987 1/1963 Harper et a1. 315-3.6

HERMAN KARL SAALBACH, Primary Examiner.

R. D. COHEN, Assistant Examiner. 

1. A SYSTEM OF ELECTRODES ARRANGED IN A MAGNETIC FOCUSING FIELD OF A VELOCITY MODULATED TUBE HAVING AN ION GETTER PUMP, WHEREIN PARTS OF THE ELECTRODE SYSTEM, WITH THE POTENTIALS THEREON AND THE MAGNETIC FOCUSING FIELD, COOPERATE JOINTLY TO EFFECT THE OPERATION OF THE GETTER PUMP, AND COMPRISING AN ION COLLECTOR FORMED BY AT LEAST TWO ELECTRODES ARRANGED EXTERNALLY OF AND SYMMETRICALLY WITH RESPECT TO THE ELECTRON BEAM PATH, SAID ELECTRODES CARRYING ESPECIALLY ON THE SURFACE THEREOF MATERIAL WHICH IS IN THE OPERATION OF THE RESPECTIVE TUBE ADAPTED TO FUNCTION AS A GETTER. 